In recent years, a carbon nanostructure, which is represented by a carbon nanotube or the like and which is formed by carbon atoms being aligned in a diameter of nanometer level, is receiving widespread attention as a carbon-base high performance material having excellent conductivity, mechanical strength and the like. As a method of producing the carbon nanotube, a pyrolysis method has been devised, where a feedstock gas such as an alcohol base gas, a hydrocarbon base gas or the like undergoes pyrolysis in a heating furnace using catalyst particles having a diameter of nanometer level, in order to grow crystalline carbon on the catalyst particles to be carbon nanotubes. The pyrolysis method includes a method where catalyst particles are carried on a base by application or the like, a method where catalysts are suspended in a vapor phase and the like.
For example, Japanese Patent Laying-Open No. 60-54998 (Patent Document 1) proposes heating a mixture of a gaseous organic transition metal compound, a carrier gas and a gaseous organic compound to 800-1300° C. for producing a vapor deposition carbon fiber in suspension.
Japanese Patent Laying-Open No. 2001-20071 (Patent Document 2) proposes a method for synthesizing a carbon nanotube including the steps of: forming a catalytic metal film on a substrate; etching the catalytic metal film to form separated catalytic metal nanoparticles; and supplying a carbon source gas into a thermochemical vapor deposition system to grow a carbon nanotube on each of the separated catalyst metal nanoparticles by thermochemical vapor deposition, thereby forming on the substrate a plurality of aligned carbon nanotubes perpendicular to the substrate, where the step of forming separated catalytic metal nanoparticles is performed by gas etching, where an etching gas, which is one selected from the group consisting of ammonia gas, hydrogen gas and hydride gas, undergoes pyrolysis to be used.
Japanese Patent Laying-Open No. 2002-255519 (Patent Document 3) proposes a method of vapor-phase synthesizing a single-layer carbon nanotube by directing a hydrocarbon gas together with a carrier gas onto a base including a thermoresistant porous body carrying dispersed catalytic microparticles and utilizing pyrolysis of the hydrocarbon gas.
Japanese Patent Laying-Open No. 2003-292315 (Patent Document 4) proposes a method of manufacturing a carbon nanotube on a metal surface using chemical vapor deposition by heating the metal and flowing toward it a gas which serves as a carbon source, characterized in that the metal surface has fine asperities provided by growing in advance oxide microparticles on the metal surface.
Unfortunately, conventional methods such as those in Patent Documents 1 to 4 produced carbon-containing by-products such as amorphous carbon or graphite in addition to desired carbon nanotubes. They also produced carbon nanotubes with large variation in their diameter, making it difficult to manufacture uniform carbon nanotubes in a stable manner.
It is assumed that the variation among the carbon nanotubes in their shape is caused by variation of the catalyst particles in their morphology. Since it is difficult to control the shape of the catalyst particles when they are formed by means of a chemical method such as the pyrolysis method, variation in their morphology is caused. Aggregation of catalyst particles may also cause a morphological variation.
Further, varying growth rate of crystalline carbon on catalyst particles as well as simultaneous occurrence of dissolution and precipitation of the carbon grown by pyrolysis of a feedstock gas on the same surface also tend to cause the variation among the resulting carbon nanotubes in their shape.
Patent Document 1: Japanese Patent Laying-Open No. 60-54998
Patent Document 2: Japanese Patent Laying-Open No. 2001-20071
Patent Document 3: Japanese Patent Laying-Open No. 2002-255519
Patent Document 4: Japanese Patent Laying-Open No. 2003-292315